Proton-exchange rates in solid tropolone as measured via carbon-13 CP/MAS NMR

On the role of experimental imperfections in constructing (1)H spin diffusion NMR plots for domain size measurements

We discuss the precision of 1D chemical-shift-based (1)H spin diffusion NMR experiments as well as straightforward experimental protocols for reducing errors. The (1)H spin diffusion NMR experiments described herein are useful for samples that contain components with significant spectral overlap in the (1)H NMR spectrum and also for samples of small mass (<1mg). We show that even in samples that display little spectral contrast, domain sizes can be determined to a relatively high degree of certainty if common experimental variability is accounted for and known. In particular, one should (1) measure flip angles to high precision (≈±1° flip angle), (2) establish a metric for phase transients to ensure their repeatability, (3) establish a reliable spectral deconvolution procedure to ascertain the deconvolved spectra of the neat components in the composite or blend spin diffusion spectrum, and (4) when possible, perform 1D chemical-shift-based (1)H spin diffusion experiments with zero total integral to partially correct for errors and uncertainties if these requirements cannot fully be implemented. We show that minimizing the degree of phase transients is not a requirement for reliable domain size measurement, but their repeatability is essential, as is knowing their contribution to the spectral offset (i.e. the J1 coefficient). When performing experiments with zero total integral in the spin diffusion NMR spectrum with carefully measured flip angles and known phase transient effects, the largest contribution to error arises from an uncertainty in the component lineshapes which can be as high as 7%. This uncertainty can be reduced considerably if the component lineshapes deconvolved from the composite or blend spin diffusion spectra adequately match previously acquired pure component spectra.

Structural analysis of a banana-liquid crystal in the B4 phase by solid-state NMR

In this paper, we present a structural investigation of 1,3-phenylene bis[4-((4-10-decyloxyphenyl)iminomethyl)-benzoate], known as a banana-liquid crystal, in the B4 phase, which was performed by solid-state nuclear magnetic resonance (NMR) methodology combined with quantum chemical calculations. The present solid-state NMR measurements including (13)C CPMAS, 2D TOSS-deTOSS, dipole-dephase, 1D and 2D EXSY, and MAS-j-HMQC provided accurate spectral assignments and unambiguous NMR parameters such as (13)C chemical shift tensors, which were used for construction of the three-dimensional structure with the aid of density functional theory calculations. In the obtained molecular structure, two arms of the bent-core molecule are asymmetrically expanded such that the direction of the dipole moment is off alignment with respect to the middle line of the center benzene ring. It is this antisymmetric structure that is the origin of the twisted helical system in the B4 phase.

Dynamic nuclear polarization study of inhibitor binding to the M2(18-60) proton transporter from influenza A

We demonstrate the use of dynamic nuclear polarization (DNP) to elucidate ligand binding to a membrane protein using dipolar recoupling magic angle spinning (MAS) NMR. In particular, we detect drug binding in the proton transporter M2(18-60) from influenza A using recoupling experiments at room temperature and with cryogenic DNP. The results indicate that the pore binding site of rimantadine is correlated with previously reported widespread chemical shift changes, suggesting functional binding in the pore. Futhermore, the (15)N-labeled ammonium of rimantadine was observed near A30 (13)Cβ and G34 (13)Cα, suggesting a possible hydrogen bond to A30 carbonyl. Cryogenic DNP was required to observe the weaker external binding site(s) in a ZF-TEDOR spectrum. This approach is generally applicable, particularly for weakly bound ligands, in which case the application of MAS NMR dipolar recoupling requires the low temperatures to quench dynamic exchange processes. For the fully protonated samples investigated, we observed DNP signal enhancements of ~10 at 400 MHz using only 4-6 mM of the polarizing agent TOTAPOL. At 600 MHz and with DNP, we measured a distance between the drug and the protein to a precision of 0.2 Å.

Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid

The proton tautomerism of pyrazole-4-carboxylic acid (PCA) has been studied by a combination of 15N CPMAS and 2H NMR spectroscopy and relaxometry. Down to 250 K, PCA forms a hydrogen bonded ribbon where adjacent carboxylic and pyrazole groups are linked by an OH···N and an O···HN hydrogen bond, forming either the tautomeric state A or B. Down to about 250 K, the tautomerism is fast on the NMR timescale and degenerate, corresponding to a phase exhibiting dynamic proton disorder. At lower temperatures, a transition to an ordered phase is observed with localized protons, assigned to an all-syn conformation adopting the sequence of tautomeric states ..ABABA.. The longitudinal 15N relaxation times T 1 of PCA-15N2 have been measured at 9.12 MHz (2.1 T). Because of the low field, a chemical shift anisotropy mechanism could be neglected, and the data were analyzed in terms of a dipolar 1H-15N relaxation mechanism, yielding the rate constants k HH. The rate constants k HD and k DD were obtained from the measurement and analysis of the 2H T 1 values of PCA-15N1-d0.9 and PCA-15N1-d0.1 measured at 46.03 MHz. Within the margin of error, no kinetic isotope effects could be detected, in contrast to previous results reported for the very fast tautomerism of solid benzoic acid dimers and the much slower tautomerism of solid 3,5-diphenyl-4-brompyrazole (DPBrP) dimers. The Arrhenius curves of all three systems were simulated using the Bell–Limbach tunneling model. Evidence for a major heavy atom motion for the tautomerism of PCA is obtained, associated with small angle reorientation of PCA molecules around the molecular axis. The observed proton order-disorder transition and the mechanism of the observed rate process are discussed in terms of a coupling of adjacent tautomeric states.

Radiation-induced synthesis of gold nanoparticles within lamellar phases. Formation of aligned colloidal gold by radiolysis

A sheared lamellar phase has been used as a nanoreactor for the synthesis of gold nanoparticles by radiolysis and by a photochemical approach. A gold salt solution (KAuCl4, 10(-2) M) is introduced into the aqueous compartments between the lipid-based bilayers. Gold nanoparticles grow within the lamellar phase as shown by TEM analysis and X-ray diffraction, limiting the particle size. Homodisperse, 2.4- and 5.9-nm-sized, spherical nanoparticles are produced by gamma irradiation and UV irradiation, respectively. When produced by radiolysis, they are perfectly aligned along the lamellae. Their UV-vis spectra display a maximum at 565 nm suggesting that nanoparticles are coupled by dipole-dipole interactions within the lamellar phase.

Dynamics of 9-hydroxyphenalenone studied by one-dimensional solid-state spin exchange NMR

We present a new NMR method to clarify the dynamics of proton tautomerism in solid 9-hydroxyphenalenone. Two 13C resonance lines influenced by the proton tautomerism have a chemical-shift difference between them, which increases with decreasing temperature. To depict the precise potential curve of the proton tautomerism, the chemical-shift difference when the proton tautomerism is completely frozen is necessary. For solid 9-hydroxyphenalenone and its derivatives, the freezing temperatures are often under -100 degrees C. When the freezing temperatures are below the temperature range in which standard magic angle spinning NMR probes can perform a sample spinning, it is very difficult to obtain the shift difference. The NMR experiments based on this new method are performed at a temperature significantly higher than -100 degrees C at which the proton tautomerism is still active. The new method yields the 13C spin relaxation rates, the rates for the proton tautomerism, and the populations of the two tautomers. Using the populations and the 13C chemical-shift difference at that temperature, we determined the chemical-shift difference at the freezing temperature. We also obtained several parameters characterizing the potential profile for the proton dynamics in solid 9-hydroxyphenalenone.

UV−Visible and 1H or 13C NMR Spectroscopic Studies on the Specific Interaction between Lithium Ions and the Anion from Tropolone or 4-Isopropyltropolone (Hinokitiol) and on the Formation of Protonated Tropolones in Acetonitrile or Other Solvents

The specific interaction between lithium ions and the tropolonate ion (C(7)H(5)O(2)-: L-) was examined by means of UV-visible and 1H or 13C NMR spectroscopy in acetonitrile and other solvents. On the basis of the electronic spectra, we can propose the formation of not only coordination-type species (Li+(L-)2) and the ion pair (Li+L-) but also a "triple cation" ((Li+)2L-) in acetonitrile and acetone; however, no "triple cation" was found in N,N-dimethylformamide (DMF) and in dimethylsulfoxide (DMSO), solvents of higher donicities and only ion pair formation between Li+ and L- in methanol of much higher donicity and acceptivity. The 1H NMR chemical shifts of the tropolonate ion with increasing Li+ concentration verified the formation of (Li+)2L- species in CD3CN and acetone-d6, but not in DMF-d6 or CD(3)OD. With increasing concentration of LiClO(4) in CD(3)CN, the 1H NMR signals of 4-isopropyltropolone (HL') in coexistence with an equivalent amount of Et(3)N shifted first toward higher and then toward lower magnetic-fields, which were explained by the formation of (Li+)(Et(3)NH+)L'- and by successive replacement of Et(3)NH+ with a second Li+ to give (Li+)2L'-. In CD(3)CN, the 1,2-C signal in the 13C NMR spectrum of tetrabutylammnium tropolonate (n-Bu(4)NC(7)H(5)O) appeared at an unexpectedly lower magnetic-field (184.4 ppm vs TMS) than that of tropolone (172.7 ppm), while other signals of the tropolonate showed normal shifts toward higher magnetic-fields upon deprotonation from tropolone. Nevertheless, with addition of LiClO(4) at higher concentrations, the higher and lower shifts of magnetic-fields for 1,2-C and other signals, respectively, supported the formation of the (Li+)2L- species, which can cause redissolution of LiL precipitates. All of the data with UV-visible and 1H and 13C NMR spectroscopy demonstrated that the protonated tropolone (or the dihydroxytropylium ion), H(2)L+, was produced by addition of trifluoromethanesulfonic or methanesulfonic acid to tropolone in acetonitrile. The order of the 5-C and 3,7-C signals in 13C NMR spectra of the tropolonate ions was altered by addition of less than an equivalent amount of H+ to the tropolonate ion in CD(3)CN. Theoretical calculations satisfied the experimental 13C NMR chemical shift values of L-, HL, and H(2)L+ in acetonitrile and were in accordance with the proposed reaction schemes.

Quantitative analysis of 17O exchange and T1 relaxation data: application to zirconium tungstate

The theoretical basis behind a recent quantitative analysis of 17O exchange in ZrW2O8 [M.R. Hampson, J.S.O. Evans, P. Hodgkinson, J. Am. Chem. Soc. 127 (2005) 15175-15181] is set out. Despite the complexities of combining the multi-exponential relaxation of half-integer quadrupolar nuclei with chemical exchange, it is shown how magnetisation transfer experiments can be analysed to obtain estimates of absolute exchange rates. The multi-exponential relaxation is best modelled using a magnetic mechanism, i.e. the rapid T1 relaxation observed, particularly at high temperatures, can be directly related to the relatively high degree of 17O labelling employed. The combination of the 1D EXSY results with T1 values as a function of temperature provides exchange rates and activation barriers over a wide temperature range (40-226 degrees C).

Synthesis, characterization and optical properties of silver and gold nanowires embedded in mesoporous MCM-41

Uniform nanowires of silver and gold inside the channels of MCM-41 were prepared by controlled reduction of their respective metal salts with sodium borohydride (NaBH4). Presence of nanowires of silver and gold in MCM-41 were confirmed by high angle X-ray diffraction (XRD) data (peaks between 2ϑ = 30 − 60°) and transmission electron microscopy (TEM) confirmed the diameter of the nanowires. Diameter of nanowires is found to be ∼ 2.8 nm which is coincident with channel diameter of MCM-41. Optical properties of these heterostructured materials Ag-MCM-41 and Au-MCM-41 reveals the presence of surface plasmon absorption peaks of silver and gold respectively, and the shift in the absorption bands are associated to agglomeration of clusters inside the channels. Room temperature photoluminescence spectra exhibits interesting optical properties as observed for direct band gap semiconductors. Non-linear optical properties (NLO) corresponding to second harmonic generation (SHG) values were also recorded for self supported films of these heterostructured materials. Enhanced optical non-linearity was found to be arising from a corresponding increase of local field near the surface plasmon resonance. Further enhancement in SHG was found with poling due to an induction of orientation order.

Rapid assignment of solution 31P NMR spectra of large proteins by solid-state spectroscopy

The application of the (31)P NMR spectroscopy to large proteins or protein complexes in solution is hampered by a relatively low intrinsic sensitivity coupled with large line widths. Therefore, the assignment of the phosphorus signals by two-dimensional NMR methods in solution is often extremely time consuming. In contrast, the quality of solid-state NMR spectra is not dependent on the molecular mass and the solubility of the protein. For the complex of Ras with the GTP-analogue GppCH(2)p we show solid-state (31)P NMR methods to be more sensitive by almost one order of magnitude than liquid-state NMR. Thus, solid-state NMR seems to be the method of choice for obtaining the resonance assignment of the phosphorus signals of protein complexes in solution. Experiments on Ras.GDP complexes show that the microcrystalline sample can be substituted by a precipitate of the sample and that unexpectedly the two structural states observed earlier in solution are present in crystals as well.

NMR studies of organic polymorphs and solvates

This review article describes the applications of NMR to the study of polymorphs and related forms (solvates) of organic (especially pharmaceutical) compounds, for which it is of increasing academic and practical importance. The nature of the systems covered is briefly introduced, as are the techniques constituting solid-state NMR. The methodologies involved are then reviewed under a number of different headings, ranging from spectral editing through relaxation times to shielding tensors and NMR crystallography. In each case the relevant applications are described. Whilst most studies concentrate on structural matters, motional effects are not neglected. A special section discusses studies of solvates (especially hydrates), and another reviews quantitative analysis.

Improved pulse sequences for pure exchange solid-state NMR spectroscopy

Spin-exchange experiments are useful for improving the resolution and establishment of sequential assignments in solid-state NMR spectra of uniformly (15)N-labeled proteins oriented macroscopically in phospholipid bilayers. To exploit this advantage fully, it is crucial that the diagonal peaks in the two-dimensional exchange spectra are suppressed. This may be accomplished using the recent pure-exchange (PUREX) experiments, which, however, suffer from up to a threefold reduction of the cross-peak intensity relative to experiments without diagonal-peak suppression. This loss in sensitivity may severely hamper the applicability for the study of membrane proteins. In this paper, we present a two-dimensional exchange experiment (iPUREX) which improves the PUREX sensitivity by 50%. The performance of iPUREX is demonstrated experimentally by proton-mediated (15)N-(15)N spin-exchange experiments for a (15)N-labeled N-acetyl-L-valyl-L-leucine dipeptide. The relevance of exchange experiments with diagonal-peak suppression for large, uniformly (15)N-labeled membrane proteins in oriented phospholipid bilayers is demonstrated numerically for the G-protein coupled receptor rhodopsin.

Solid-state nuclear magnetic resonance spectroscopy--pharmaceutical applications

Solid-state nuclear magnetic resonance (NMR) spectroscopy has become an integral technique in the field of pharmaceutical sciences. This review focuses on the use of solid-state NMR techniques for the characterization of pharmaceutical solids (drug substance and dosage form). These techniques include methods for (1) studying structure and conformation, (2) analyzing molecular motions (relaxation and exchange spectroscopy), (3) assigning resonances (spectral editing and two-dimensional correlation spectroscopy), and (4) measuring internuclear distances.

One- and two-dimensional 15N exchange CP/MAS NMR studies of the structure and electronic properties of the intermolecular N-H...N hydrogen bond in imidazole crystal

The hydrogen bond of the type N-H...N in imidazole crystal has been studied by one and two-dimensional 15N exchange CP/MAS NMR measurements as well as the powder NMR spectrum. The chemical shift anisotropies for -N= and -N< were determined from the powder 1D spectrum. In 2D exchange CP/MAS NMR spectrum, the cross peaks between the 15N main resonance peaks for -N= and -N< were observed, implying that magnetization exchange between -N= and -N< takes place. The 1D exchange CP/MAS NMR measurements determined the exchange rate of magnetization at 289 K to be 1.3 and 1.5 s(-1) for -N= and -N<, respectively. The proton-driven spin-diffusion model interprets the experimental values, and the exchange rate depends strongly on the RF power of the proton decoupling field, suggesting that the magnetization transfer between -N= and -N< takes place by the 1H-driven spin-diffusion mechanism.

PATROS-A new MAS exchange method using sideband separation: application to Poly(n-butylmethacrylate)

The tr-ODESSA method (Reichert et al., J. Magn. Reson. 125, 245 (1997)), which is a 1D MAS experiment designed to monitor spin exchange involving both equivalent and inequivalent sites, is extended to situations where the spectrum consists of several spinning side band (ssb) manifolds with small chemical shift anisotropies. To increase the spectral resolution in such situations, the tr-ODESSA sequence is combined with that of PASS to a single experiment, which we term PATROS. In this hybrid experiment, magnetization transfer is monitored by the tr-ODESSA part, while the increase in resolution is provided by the separation of the ssb according to their order, during the PASS part. We demonstrate the feasibility of the method on a standard solid dimethylsulfone (DMS) sample and then apply it to monitor separately the ultraslow motions of the main- and side-chains in the polymer poly(n-butylmethacrylate). Theoretical expressions for the ssb intensities in PATROS experiments are derived and the merits and limitations of the method are discussed. Copyright 2000 Academic Press.

Initial conditions for carbon-13 MAS NMR 1D exchange involving chemically equivalent and inequivalent nuclei

A major problem in dynamic 1D (13)C MAS NMR concerns the exchange between magnetically inequivalent, but chemically equivalent sites, whose signals are not resolved in the regular 1D spectrum. This difficulty may be overcome by properly preparing the initial nonequilibrium state of the spin system in the exchange experiments. In the present paper we discuss the advantages and limitations of several such experiments already in use and propose a new sequence, which we term SELDOM-ODESSA. Unlike the other 1D-exchange methods, this experiment yields pure absorption spectra that can more readily be analyzed quantitatively. The experiment is a hybrid comprising a SELDOM sequence, for selective excitation of one of the spinning sideband manifolds in the spectrum, followed by the ODESSA sequence, which induces alternate polarization in the excited sideband manifold. The evolution of the spectrum following this sequence provides information on both the exchange between congruent sites belonging to the same group of equivalent nuclei, and the exchange between inequivalent sites. Results are presented for a tropolone sample specifically enriched in carbon-13 at the carbonyl and hydroxyl sites. The dominant exchange mechanism in this sample involves spin diffusion. The various spin exchange processes in this sample, in the presence and absence of proton decoupling during the mixing time, are measured and discussed. Copyright 2000 Academic Press.

Pure-exchange solid-state NMR

Three exchange nuclear magnetic resonance (NMR) techniques are presented that yield (13)C NMR spectra exclusively of slowly reorienting segments, suppressing the often dominant signals of immobile components. The first technique eliminates the diagonal ridge that usually dominates two-dimensional (2D) exchange NMR spectra and that makes it hard to detect the broad and low off-diagonal exchange patterns. A modulation of the 2D exchange spectrum by the sine-square of a factor which is proportional to the difference between evolution and detection frequencies is generated by fixed additional evolution and detection periods of duration tau, yielding a 2D pure-exchange (PUREX) spectrum. Smooth off-diagonal intensity is obtained by systematically incrementing tau and summing up the resulting spectra. The related second technique yields a static one-dimensional (1D) spectrum selectively of the exchanging site(s), which can thus be identified. Efficient detection of previously almost unobservable slow motions in a semicrystalline polymer is demonstrated. The third approach, a 1D pure-exchange experiment under magic-angle spinning, is an extension of the exchange-induced sideband (EIS) method. A TOSS (total suppression of sidebands) spectrum obtained after the same number of pulses and delays, with a simple swap of z periods, is subtracted from the EIS spectrum, leaving only the exchange-induced sidebands and a strong, easily detected centerband of the mobile site(s).

MAS NMR studies of carbon-13 spin exchange in durene

One- (1-D) and two-dimensional (2-D) carbon-13 NMR exchange measurements in powder samples of isotopically normal durene under magic angle spinning (MAS) are reported. The experiments include rotor synchronized 2-D exchange (RS2DE), 1-D magnetization transfer (MT) and time reverse ODESSA (tr-ODESSA). The latter two experiments were performed as a function of several external parameters, including proton decoupling field during mixing time, sample spinning rate and partly, of temperature. The effects of these parameters on the spin exchange induced by spin diffusion and by chemical, or physical exchange, is discussed. Spin exchange between all types of carbons in the durene molecules occurs on the time scale of seconds. From the dependence of the spin exchange rate on the external parameters it is concluded that the process is dominated by spin diffusion. On the basis of these results an upper limit of 10(-16) cm2 s(-1) can be set for the self-diffusion constant in crystalline durene.

Dynamic Hydrogen Disorder in Solid Tropolone. A Single-Crystal NMR Study of the Hydroxyl Deuterons

The orientation and temperature dependences of the deuterium NMR spectrum and spin-lattice relaxation time of the hydroxyl deuterons in single crystals of tropolone-d 1 are reported. The results are interpreted in terms of a dynamic hydrogen disorder model in which the hydrogen nuclei move in an asymmetric double well potential. According to this model, the hydrogen-bonded dimer structure as determined by X-ray diffraction constitutes a majority species in the tropolone crystal, comprising more than 98% of the molecules at room temperature. However, there also exists a tautomeric minority species formed by a concerted back and forth shifting of the hydroxyl hydrogens (deuterons) along the hydrogen bonds to the nearby carbonyl oxygens. This process results in a modulation of the electric field gradient tensor at the site of the deuterons, thus providing an efficient relaxation mechanism. The concentration of the minority species is too low and its lifetime is too short to make its direct observation possible. Still, structural information about this species and kinetic and thermodynamic parameters of the hydrogen shift process can be derived by fitting the measured T 1 values to the above model.

New Aspects of Spin Diffusion and Cross Relaxation in Solid-State NMR

Theoretical models for motionally driven spin diffusion are compared and applied to spins S = (1/2) and S = 1. Their predictions are quantitatively corroborated by experiments on a single crystal of deuterated biphenyl. In this system, a molecular flip process drives spin diffusion and makes it strongly temperature dependent. In the second part, it is shown how spin diffusion of quadrupolar order degenerates into cross relaxation. A single crystal of partially deuterated durene provides a clear-cut example of cross relaxation in a dipolar coupled pair of spins S = 1. This observation explains why, in solids, the relaxation time of quadrupolar order, T 1Q , is often much shorter than T 1 .

Solid-state NMR studies of 1H spin diffusion in adsorbed organic molecules

1H spin diffusion times of toluene (MB) and tetrahydrofuran (THF) adsorbed on a series of porous solids (charcoal, SiO2 and Al2O3) were measured by a selective inversion technique. The experimental results show that they cover a wide range (from less than one millisecond to several hundreds of milliseconds). For all samples, a tri-exponential behavior was observed in the magnetization recovery processes of the negative peaks. This is attributed to the existence of the two different kinds of spin diffusion processes in addition to the T1 relaxation. One is assigned to the intermolecular spin diffusion between the surface acidic protons of the adsorbent and the organic molecules adsorbed on the solid surface, the other to the intramolecular spin diffusion of adsorbed molecules. Due to hydrogen bonding between the surface hydroxyl groups and the adsorbate, the intermolecular spin diffusion of THF adsorbed on various solids is more effective compared to that of adsorbed MB. In addition, the intermolecular 1H spin diffusion between charcoal and adsorbed THF molecules was confirmed by indirect measurement suggested by Tekely et al.